Field Calibrations in Practice: Results from Three Years of Weekly Acoustic Surveys of Nearshore Fish Populations in Lake Ontario

Standard calibration techniques are necessary for acoustic estimates of fish abundance to ensure reliable system performance over time and for comparison of results to other studies.   Standardization of calibration procedure among acoustic studies should consider: how often field calibrations are performed, the identification of the appropriate distribution statistic for comparing empirical and reference target strengths (TS), and the methodology for obtaining that statistic.  The effect of analytical methods and frequency of performing calibrations on fish abundance estimates was explored using field-calibration data from three years of repeated acoustic surveys.  A 2.5 km² nearshore area of Lake Ontario (3 to 36 m water depth) was sampled twice along twenty 1-km parallel transects by a Simrad EK60 echosounder system with a pair of 70- and 200-kHz split-beam transducers pointed downward and another pair pointed horizontally.   This survey was repeated during one day and one night of each week during June-December 2009, April-November 2010, and April-December 2011.  Prior to each survey, the echosounder system was calibrated by the Simrad standard target method using a 38.1 mm tungsten carbide sphere.  The changes in transducer gain and TS deviations from the reference TS over time within and among transducers were quantified.  Additionally, the effect of processing techniques (Simrad ER60 calibration module and Echoview software) on the acoustic measure of fish abundance was estimated.